Pearls have always been with us. Mrs Thatcher wore pearls a lot, as earrings and necklaces of graduated beads. Post Thatcher they were for so long so naff that even the Queen cut down on the yardage and tonnage of pearls to be slung around her neck on ceremonial occasions. Diana, Princess of Wales, festooned herself in pearls. She was so keen on pearl chokers that she had them in every variation from single-strand to 11-strand. She usually wore her chokers too tight so that they sat halfway up her neck. Her successor as spouse to Prince Charles errs in the opposite direction, wearing huge horse-collars of pearls that give her all the dignity and authority of a pearlie queen, and ruin the hang of all her dress-and-coat sets. With the market at present flooded by immense quantities of Chinese freshwater pearls, the cachet of the pearl could be said to have entered upon terminal eclipse. That was before Condoleezza Rice and the advent of the power pearl.

Condie is the consummate power-dresser. Every pant suit - whether by St John, Armani or Versace, in black, navy or charcoal grey - is immaculate, no matter how long she has been crouched memorising briefs as she is whirled by plane from one end of the earth to another. She is said to work out at 4am each morning; certainly the belly is flat and the stride purposeful, no matter how long she has been cooped up in a plane. Her hair is regularly beaten into submission. Right now she is sporting a ringer for the Michelle Obama bob, which itself is a version of the Jackie Onassis big hair flick. Occasionally, Condie lets herself be seen in knee-high, high-heeled jackboots, to the unspeakable excitement of the armies of lackeys that follow her about. She wears dark lipstick, highlighting the expressiveness of her eyes and the occasional dazzle of her smile. Otherwise her subfusc is relieved only by the milky radiance of her power pearls.

Power pearls are pure white and large, anything from 11mm in diameter to 16mm, in a single strand, which must hang within rather than over the neckline. The size reveals that power pearls are not properly "natural". Natural pearls of that size are extremely rare, and command the kind of price only a queen could afford. Power pearls are farmed, cultured if you prefer. They can be cheap, that is, with only a thin layer of nacre, or they can be rich and glossy as Condie's are. Over the years, Condie's strings of pearls have been getting shorter and the beads bigger. Though I have kept close watch, I am not sure just how many strings she has. Cultured pearls that are not perfectly round cost much less than rounds, and can look as good or better. When a screaming Code Pink protester threatened her with bloody hands a year ago, Condie stood firm, safe behind her power pearls. When she needs special clout, she dons a particular string of big heavy pearls that lie flat to her collarbone. At the press conference on August 13, when she got stuck into Russia for bullying Georgia, frowning, glaring and snarling, the big pearls shone within the revers of her blueblack suit like moons in a starless night sky. The public was so dazzled by this Queen of the Night performance they didn't realise just how ineffectual her response actually was. Power pearls are glamour, bravado and insolence.

Power pearls have now crossed the floor of the house. For a few weeks now, Michelle Obama has been wearing power pearls and hers are even bigger than Condie's, so big that bloggers beg the cruel world to tell them why their idol's wife wears "outsize fake pearls". Michelle's not saying whether they're fake or not, and lets it go on being a grey area. She knows by what pearls do for Condie's image what they could do for hers, softening her features, lengthening her neck and giving her that gleam of class. So far, the image-meisters have not intervened and she is still wearing them. Madeleine M Kunin's new book on women in US politics is called Pearls, Politics and Power. When she came on stage to endorse Obama's campaign and belatedly declare her support, wearing the ubiquitous full-bottomed pantsuit, even Hillary Clinton had brightened up her image with a row of power pearls.

Despite Kunin, real power and pearls (real, cultured or fake) don't go together. Angela Merkel, the most powerful woman in Europe, wouldn't be seen dead in the full-on row of pearls. I suspect that Helen Clark, prime minister of New Zealand, doesn't have a pearl to her name. Pearls are tears; Diana Spencer wore her jewelled ligatures as signifiers of subjection. Condie Rice is George Bush's creature, and when he steps down he will take her with him. The consensus is that she will not find another job in politics. Hillary has taken to wearing pearls in defeat, which leaves only Michelle.